1. We don't need to handle separately the scalar case -- tolist()
   returns a scalar when called on a 0d-array, as returned by
   np.vectorize() when called on a scalar.
2. We don't need to handle separately the empty case, as long as the
   output type ("O") is passed to np.vectorize.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way